The Sun’s outer atmosphere, known as the solar corona, is a strange and violent place.
It is a million times hotter than the Sun’s surface and filled with loops and ropes of plasma—gas made of electrically charged particles—twisted into enormous magnetic structures.
Understanding how these ropes behave is important for explaining solar flares and other explosive events in space.
Now, researchers at Caltech have discovered a surprising new state of stability that appears not only in solar plasma but also in structures spanning light-years across the galaxy.
The discovery comes from Paul Bellan, professor of applied physics, and his former graduate student Yang Zhang, who is now a NASA postdoctoral fellow.
Together, they studied magnetic “flux ropes,” which are tubes of plasma threaded with magnetic fields, like a garden hose wrapped with a spiral stripe.
These flux ropes can form in the solar corona, in laboratory plasma experiments, and even in astrophysical objects many thousands of light-years across.
In Caltech’s laboratory, Bellan and Zhang created miniature versions of solar flares inside a large vacuum chamber. Two electrodes were placed in the chamber, and coils generated a magnetic field across them.
When a high voltage was applied, the neutral gas inside was ionized into plasma. To their surprise, the plasma automatically organized itself into a braided structure: two flux ropes twisted around each other in a double helix, much like strands of DNA.
What made this observation remarkable was that the braided double helix was stable. Instead of twisting tighter or untwisting into chaos, the structure balanced itself. Zhang and Bellan realized that this stability came from a tug-of-war between attractive and repulsive magnetic forces.
When currents flow along both braided strands in the same direction, the parts of the current moving lengthwise attract each other, while the parts moving around the spiral repel each other. At a certain “critical angle” of twist, the attraction and repulsion balance perfectly, producing an equilibrium that locks the double helix into place.
Until now, scientists had assumed that two same-current flux ropes would always merge, because parallel currents are known to attract.
But earlier hints from other experiments suggested otherwise. Zhang’s new mathematical model explained the puzzle, showing how the balance of forces creates a natural equilibrium state. His equations not only described the lab experiments but could also predict the behavior of flux ropes in space.
To test the idea, Zhang applied the model to the Double Helix Nebula, a striking structure located 25,000 light-years from Earth near the center of the Milky Way. The nebula consists of two giant plasma ropes twisted together over a span of 70 light-years.
Using just two measurements—the diameter of the strands and the spacing of the twists—Zhang was able to calculate the equilibrium angle of twist. The result matched the astronomical observations.
The finding suggests that the same basic laws of plasma physics govern phenomena across an astonishing range of scales, from a few centimeters in a lab chamber to vast galactic structures. As Bellan explains, “What we see in lab experiments and in solar and astrophysical observations are governed by the same equations.”
The study, published in Physical Review Letters under the title Magnetic Double Helix, not only deepens our understanding of how magnetic fields shape the universe but also demonstrates how experiments on Earth can help explain cosmic mysteries millions of times larger.
